The use of liquid fertilizers has been known for some time. Indeed, it is reported that Sir Humphrey Davey experimented with solutions of inorganic fertilizers as early as 1808. The principal liquid fertilizers used in the U.S. usually contain at least two of the primary plant nutrients, viz. N, P.sub.2 O.sub.5, and K.sub.2 O. The use of liquid fertilizers has several advantages. Liquids are, of course, readily transportable by pumping. Further, liquids can be metered more accurately and evenly dispersed than solids. Still further, herbicides, insecticides, and other additives are readily admixed with the liquid fertilizers and hence more evenly distributed upon the surface of the soil and plants.
Potassium pyrophosphate is particularly amenable to use as a liquid fertilizer. More particularly, potassium pyrophosphate (K.sub.4 P.sub.2 O.sub.7) is 100 percent plant food (57 percent K.sub.2 O and 43 percent P.sub.2 O.sub.5). Further, potassium pyrophosphate has a high solubility in water at ambient temperatures (50-65 percent by weight) and, thus, has a lower application cost per pound than those fertilizers having a lower solubility and necessarily requiring the application of a greater volume of solution to provide an equivalent amount of plant food.
One method of producing potassium pyrophosphate proposed heretofore involves several separate steps. In the first step, potassium hydroxide is produced electrolytically by feeding potassium chloride into a mercury cell to produce an aqueous solution of potassium hydroxide. In the second step, the solution of potassium hydroxide is reacted with aqueous phosphoric acid to form a solution of di-potassium orthophosphate (K.sub.2 HPO.sub.4). The water then is evaporated from the di-potassium orthophosphate and the orthophosphate is calcined at approximately 750.degree.C to produce potassium pyrophosphate.
A disadvantage of this method is that the capacity is fixed by the size or number of electrolytic cells available.
More recently in U.S. Pat. No. 3,361,523, a two-step method of preparing various crystalline condensed phosphates is suggested. Broadly, the method comprises reacting a condensed phosphoric acid with a sufficient amount of an alkali metal or alkaline earth metal source to form the desired product. In the first step, an amorphous intermediate condensed phosphate material is formed. That material then is ground and calcined at a temperature below the liquification temperature of the desired product for a time sufficient to effect a solid phase conversion.
A disadvantage of this method is that it requires the use of a condensed phosphoric acid as a starting material.
Both of the foregoing methods require multiple steps to produce an alkali metal pyrophosphate. Obviously, it would be desirable to have a one-step method of producing an alkali metal pyrophosphate. Further, it would be desirable to be able to use an inexpensive, readily available starting material such as, for example, wet process phosphoric acid.